Total Pageviews



Thursday, January 30, 2014

Thursday's Orthotic Discussion of the Week: Pouring the Negative Cast and Building the Anterior Platform for the Inverted Technique

Dear Dr. Blake,
Thank you very much for your blog and for this post ! I've read with great interest your post from Podiatry Arena and, as a self educated person [in my country doesn't exist podiatric schools] I want to tell you that your post is one of the best, expecially from a practical point of view !

 I have a question which maybe seems stupid for you and I apologies for this. I personnaly have difficulties with anterior platform building so I kindly ask you to give me some instructions regarding the composition of the pink plaster from photos. In my practice I make it either too fluid or too rigid and simple I didn't succeed to build the anterior platform in the way you have described here !

I'm expecting with great interest March, when you'll post your manual !
Thank you for your kindness !

Robert (name changed)

Dear Robert,
     I am always honored by getting emails and compliments, and especially from so far. Thank you so very much and I hope I can help. Here are some photos that I hope will help. More questions are encouraged.

Here is the initial set up for pouring of the negative casts to make them a solid positive cast. The most important part of the photo is my coffee cup since it is 6:30 am on a Saturday morning. One basin is for the soap solution and one basin is for the casting plaster to be used in making the positive cast. 

For 2 feet I am pouring the plaster to make a positive cast I use 5 cups of plaster and 3 cups of water. There is the 1 cup volume rubber pouring bowl. For 2 pairs of feet, 4 total, I will use 10 cups of plaster and 6 cups of water. The basin I use holds up to 15 cups of plaster. 

This is the Casting Plaster I use with 30 minute Set Time. 

It is also called Red Tag 30. 

I use a one cup bowl, but you can buy these bowls that hold 5 or 10 cups at once to make life easier at times. Here the plaster is in the bowl and will be emptied into the basin to begin the process. 

I have fillled the basin with 10 cups of plaster good enough for 2 averaged sized pairs of feet. If both your casts are for sizes 12 or greater, consider 12 or 13 cups of plaster. The water ratio is still 60% approximately the plaster amount.

I buy a soapy concentrate and dilute with water 50%. This is poured into the negative cast to saturate the bottom and sides. Any extra soap is allowed to drip out and returned to the bottle. This makes it so much easier to get off the negative casting material once the positive is dried. 

Here are the negative casts after having been soaked thoroughly with the soapy solution and then being tipped over to allow the soap to drain. 

The negative casts are then leveled so that the heel bisections stand perfectly vertical when poured. This is done with every cast so that the top of the cast represents a parallel with the ground. 

Here my talented brother Bob is pouring water in a 60% ratio to the plaster with some colored dye to make the plaster solution for pouring. One pair of casts typically need 5 cups of plaster and 3 cups of water. 

The plaster (mud!!) is mixed thoroughly to get out any clumps of plaster so that the solution is a uniform consistency. 

Plaster clumps are completely broken apart

Then the very smooth consistent liquid plaster is poured into the balanced to heel vertical negative casts.

My great brother Bob skillfully pours the plaster to make the positive casts set a heel vertical.

Here the negative casts sit after the plaster is poured

Typically you can pour up to 3 pairs of casts in one basin. That basin would have 15 cups of plaster and 9 cups of water.

Once the negative casts are poured, it is important to recheck the heel bisection to make sure it has remained vertical. 

After pouring, sticks are placed into positives to minimize breakage with the high pressure vacuum press. 

Once you remove the casting plaster, the first and fifth metatarsal areas are marked to define the anterior platform borders. You find the contact point on the first metatarsal (lowest point) and then place the line 15 mm closer to the arch. I like to then even the 5th metatarsal platform with that line, but the fifth metatarsal is normally a little shorter. The vertical lines are in the space between the first and second metatarsals and the 4th/5th metatarsals.

Here you can see the proximal line 15 mm from the contact point. This will be where the plastic of the orthotic device ends. You do not want the plastic to run under the weight bearing surface when walking or running. 

Here the nails are placed on the platforms to make the angle for Fettig Modification of the Inverted Technique. Here the medial nail under the big toe joint will set the overall inversion and lateral nail under the fifth metatarsal head for the forefoot valgus correction.

When I am working with plaster, I typically have 2 or 3 bowls going at once. The plaster has to be right, not too runny and not to solid, to apply and shape. It takes time to learn your plaster. I try not to stir it much, for stirring will make the plaster harden faster. I try to pour off the excess water once the bubbles stop, for too long with excess water makes the plaster to take forever to dry. 

I use wooden sticks soaked for 10 minutes or so for making the transition from anterior platform to the medial arch fill more solid during vacuum press. The high pressures can break the platform away from the medial arch without the sticks in place.

After the nails are in place, and the plaster the right consistency, place a piece of paper down to make the anterior platform. 

Gently stir the plaster to check the consistency

It is so important to play with the plaster and know it's consistency

and play some more!

And more

I place dye into the plaster to make the various parts of the positive cast stand out

Mix it in well

When the plaster is ready, place it on a piece of paper and press the positive cast gently down. It is important not to push too hard to distort the nails. It is important not to push too lightly and lose the angle set by the nails.

Here the anterior platform area of the positive cast is coming in contact with the plaster mound

Gently, but firmly, I press the positive down into the mud

Then I use a spatula to make the medial and lateral edges straight up and down.

It is important to check your angles, if off from what was ordered, immediately knock off the platform and start all over again.

This photo is out of order but shows the positive casts, after being poured, drying for 1 hour in the sun. 

Here the wooden stick is placed into the anterior platform to set further strengthen the area before pressing. The total length of the stick coming out of the platform and into the arch will be 1 and 1/2 inches in general.

Another view of the stick

The platform is being formed following the lines on the positive cast.

When cutting the anterior platform, keep the spatula moist with water to allow easier trimming.

Do not lose focus on the shape of the platform desired and outlined by those lines 

The medial and lateral sides will still need to be trimmed to go straight up and down from the positive cast.

Another view of the 20 degrees Inversion with 6 degree forefoot valgus in the Fettig modification of the Inverted Orthotic Technique.

Trimming of the sides with scrapper

Final product with anterior platform with medial and lateral expansions.

No comments:

Post a Comment

Thank you very much for leaving a comment. Due to my time restraints, some comments may not be answered.I will answer questions that I feel will help the community as a whole.. I can only answer medical questions in a general form. No specific answers can be given. Please consult a podiatrist, therapist, orthopedist, or sports medicine physician in your area for specific questions.